The technique uses ultrasound imaging to track the drug in the body and release it with a pop once it has reached its target.

"Imagine soap bubbles," said Natalya Rapoport of the University of Utah's Department of Bioengineering. "Now imagine a drug in the soap bubbles."

When injected in the bloodstream, these tiny bubbles loaded with the chemotherapy drug doxorubicin seek out cancer tumors and congregate.

"These nanobubbles don't penetrate normal blood vessels but they do penetrate blood vessels in the tumor," said Rapoport, whose study appears in the Journal of the National Cancer Institute.

Once in the tumor, the nanobubbles combine to form larger "microbubbles," which can be seen on an ultrasound.

"When these bubbles accumulate, I give strong ultrasound radiation to the tumor to blow them up," she said in a telephone interview. "Then the drug gets out of these bubbles locally at the tumor site."

In mice, the nanobubbles were more effective at blocking tumor growth than other nanoparticle delivery methods.

"So far, we tried it in mice with human breast cancer tumors. We got very promising results," she said.

Rapoport will test the therapy in larger animals and hopes to start human clinical trials in three years.